Electric furnace

ABSTRACT

Particulate metal in flowable form or liquid metal is continually dispensed on to the charge in the furnace vessel through a tubular non-consumable electrode constituting a reservoir for this metal. The metal discharges through a nozzle on the tip of the electrode and an arc is sustained in the metal flowing between this nozzle and the charge surfaces. In the case of particulate metal this would normally comprise only part of the total metal charge ultimately in the furnace, unless the application is in respect of vacuum arc re-melting or refining where it would constitute the whole charge. Where liquid metal is employed this only constitutes a small proportion of the total charge. In the latter embodiment starting presents no problem since a conductive path is provided but, with particulate metal, where a conductive path is not provided initially, starting is effected with the aid of an auxilliary electrode.

United States Patent 91 Bowman 3,736,359 May 29, 1973 PrimaryExaminer-Roy N. Envall, Jr. Attorney-Bacon & Thomas [57] ABSTRACTParticulate metal in flowable form or liquid metal is continuallydispensed on to the charge in the furnace vessel through a tubularnon-consumable electrode constituting a reservoir for this metal. Themetal discharges through a nozzle on the tip of the electrode and an arcis sustained in the metal flowing between this nozzle and the chargesurfaces. In the case of particulate metal this would normally compriseonly part of the total metal charge ultimately in the furnace, unlessthe application is in respect of vacuum are remelting or refining whereit would constitute the whole charge. Where liquid metal is employedthis only constitutes a small proportion of the total charge. In thelatter embodiment starting presents no problem since a conductive pathis provided but, with particulate metal, where a conductive path is notprovided initially, starting is effected with the aid of an auxilliaryelectrode.

17 Claims, 2 Drawing Figures PATENTED MAY 2 9 I975 IONISABLE SEEDEDFLUID REDUGING GAS FIG. 7.

ELECTRIC FURNACE This invention relates to a method of and apparatus forsteel making in an electric furnace and to electric steelmakingfurnaces, e.g. arc furnaces or electrode remelting furnaces, and moreparticularly relates to a method of melting a metal charge and theelectrode employed in this method.

Arc furnaces of the direct type in which an arc is struck between agraphite electrode and the charge are conventionally ignited by movingthe electrode into contact with the metal charge e.g. steel scrap, inthe vessel so as to create a short-circuit and then the electrode iswithdrawn upwards, drawing an arc with it. Thereafter, the charge ismelted by the heat generated and the electrode is slowly consumed duringthis process.

Several disadvantages are attendent on this manner of operation howeversince, quite apart from the obvious disadvantage of having the electrodeconsumed (which is an expensive item) and possibly fractured on strikingthe charge, the electrical power input is low before a consistent arc isobtained and the arc may even then run in an unstable manner and localhot spots may be realised eroding the refractory-lined walls.

It is an object of this invention to mitigate these drawbacks inrelation to are furnaces but it is to be understood that, as mentioned,the invention is also relevant in vacuum arc electrode re-melting orrefining. In particular, in the latter scheme an arc is struck between ametal electrode and a starter block (constituting an initial charge) ina crucible in an evacuated atmosphere and the heat from the arcprogressively melts the electrode which is thus consumed andre-constituted in the I molten metal pool in the crucible. The metalre-melted and re-constituted in this fashion has fewer impurities andbetter properties than the starting electrode by reason of the removalof impurities under vacuum.

With the present invention, such remelting can be performed by a newmethod which avoids'the need for a consumable electrode, as such, andensures a stable arc, reducing the risk of crucible wall erosion.

According to one aspect, the present invention provides a method ofsteelmaking by melting a charge in an electric furnace,.in whichparticulate or liquid metal is continually dispenses on to the chargethrough a noz- 216 on the tip of a tubular non-consumable electrode, anare being sustained in the metal flowing between the electrode nozzleand the surface of the charge.

The invention also provides apparatus for performing this method, thetubular electrode constituting a replenishable reservoir for the metaland being movable away from and towards the charge through the roof ofthe furnace.

The electrode is preferably madefrom a conductive metal, butalternatively it may be insulated and simply embody a-conductive pathfor conducting current to the metal the nozzle which may be made from arefractory material.

The operating and physical parameters, e.g. the applied voltage and thecurrent flow, the dimensions of the nozzle aperture, the properties ofthe metal and its flow rate etc., are chosen such as to achieve abalanced condition in which the arc runs from just below the nozzle tipto tthe charge surface. In this manner the nozzle tip is not subjectedto such a harsh environment and erosion is less severe than wouldotherwise be the case.

Molten steel is preferably employed in the liquid embodiment and ifparticulate metal is employed steel balls or pre-reduced iron pelletsmay be used, the latter being particularly advantageous since they havegood electrical conductivity and the higher the conductivity the less isthe chance of them welding together at the nozzle aperture.

With the liquid case the amount of liquid metal discharged through thenozzle is only very small in relation to the size of the initial chargeentered in the vessel, at least in so far as its use in arc furnaces isconcerned, but with the particulate metal case, particularly wherepre-reduced iron pellets are used, these pellets may ultimately compriseabout half the total vessel charge depending on the scrap/pellet ratiodesired in the furnace and, of course, will comprise the whole of there-constituted charge in the case of re-melting.

With the liquid case starting presents no problem but when theparticulate metal is initially dispensed through the nozzle there is nocontinuous conductive path between this metal in the reservoir and thecharge surface. Thus, in this instance it is convenient to effectstarting by using an auxillary electrode, e.g. a graphite rod; this rodmay be projected through the nozzle aperture on to the charge and thenwithdrawn, drawing the arc with it which is then sustained in thefalling stream of particulate metal. The rod may then be used forregulating the flow.

Alternatively, starting may be effected by the use of a fluid seededwith a compound which is ionised at the subsisting temperature so as topromote breakdown of the gap, e.g. as described in our co-pending patentapplication Ser. No. 215,3l0.

In addition, oxidation of the particulate metal may be minimised bypassing a reducing gas through the electrode, and further, a gaseous orliquid fuel may be used to supplement the heat reaction in the bath.

In order that the invention may be fully understood, two embodimentsthereof will now be described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a sectional side elevation through part of an arc furnaceschematically illustrating this invention, 1

with liquid steel in the tubular electrode; and

FIG. 2 is a sectional side elevation similar to FIG. 1 with metalpellets in the tubular electrode.

Referring now to FIG. 1, an electrode 3 supported by an arm 4 extendsthrough the roof 5 of an arc furnace vessel charged with steel scrap 6.The electrode is of tubular construction and includes a graphite sleeve7 in an inner and outer refractory heat insulating casing 8, theelectrode tapering at its tip to terminate in a refractory nozzle 9having a circular orifice.

The graphite sleeve is connected to a current supply source andprovision is made for a conductive path through the inner refractorycasing 8 e.g., by apertures 8a in this casing or conductive pins 8bprojecting therethrough so as to complete a current path to molten steel10 which partly fills the tubular electrode, this reservoir of steelbeing continually topped-up from an external source (not shown). Thescrap metal '6 completes the electric circuit in the normal way.

In operation, the liquid steel is introduced into the electrode, thepower is switched-on and as the jet (ll) of molten metal issues from thenozzle an are 12 is struck and sustained in the flowing metal, meltingthe charge beneath.

The dimensions of the orifice and the resulting flow rate of the moltenmetal are such that a balanced flow is obtained by which the arc risesto a point just below the nozzle orifice too fine a jet would boil andtoo low a flow rate would tend to cause the arc to extend right up tothe nozzle orifice, ultimately destroying it.

The molten jet is maintained throughout the melt down simply so as tosustain the arc; the percentage of this metal which is reconstituted inthe bath is negligible compared with the capacity of the latter.

Referring now to FIG. 2, a tubular steel electrode 13 is supported fromabove as before. This electrode is again tapered at its tip and isterminated in a refractory nozzle 14 having a circular orifice.

Mounted centrally within the electrode and reciprocably 'movable throughthe orifice is a circular-section rod 15 of graphite, this rod beingused for starting and flow regulation in a manner to be described.

In operation, the rod 15 is initially positioned to seal the orifice andparticulate metal 16, e.g., in the form of steel balls or pre-reducediron pellets, is charged into the electrode. This metal is preferablyspheroidal so as to flow easily. The electrode is then lowered towardsthe scrap metal charge 6 until it lies in close proximity thereto andthen the graphite rod is moved down through the nozzle into contact withthis charge creating a short-circuit, and then withdrawn, drawing an arcwith it.

Continued withdrawal of the rod then exposes the aperture in the nozzlewhereupon the particulate metal flows through and the are 77 issustained in this flow of metal. Thereafter the rod can be moved toregulate the flow, particulate metal continuously being charged into theelectrode so as to maintain a constant reservoir.

In practice, the flow rate and the electrode spacing are adjusted sothat once the arc has been transferred from the rod to the massed metalparticles at the mouth of the nozzle, it is thereafter sustained in thisflowing metal to a point just below the nozzle orifice, as in theprevious embodiment.

Thecomsumption of particulate metal may be very high; the particlesre-constituted in the melt may in fact comprise the greater part'of theultimate vessel charge.

In this regard, compared with the current practice frequently employedby which such pellets are dispensed on to the scrap to supplement thecharge, this invention offers significant advantages because whereasthese pellets are dispensed on to the surface of the melt, i.e. on tothe slag layer where they oxidise before combining, with this inventionthey are jetted" directly into the melt, penetrating the slag layer. Inaddition, no additional aperture is required in the roof of the vesselfor introducing these pellets which minimises fume effluent problems. 7

Clearly, accurate control of the flow rate at the nozzle is of primeimportance in order to maintain the desired are parameters and it mustbe ensured that the current density at the orifice is not so high as toweld the particles together at this point; welding may also be realisedif the electrical conductivity of the particles is too low, i.e. if theyhave a high contact resistance. Care must also be taken to avoid theparticles forming a natural bridge amongst themselves at the orifice; inthis regard, it has been found that for a circular aperture the diameterof the orifice should be at least eight times the mean diameter of theparticles for reliable flow.

As mentioned above, oxidation of the particles in the electrode may beminimised by passing a reducing gas through them, e.g., as exemplifiedby the arrow 18, and furthermore as an alternative to starting with therod 15, starting may be effected by projecting a seeded" fluid into thegap through an auxiliary lance 19.

Each of the embodiments described is suitable for either a.c. or doenergisation and either single or multiple electrodes, e.g. three forthree-phase operation, may be used.

We claim:

1. A method of steelmaking by melting a charge in an electric furnace,comprising: continually dispensing a flowable metal on to the chargethrough a nozzle on the tip of a tubular, non-consumable electrode, andstriking and sustaining an arc in the metal flowing between theelectrode nozzle and the surface of the charge thereby melting thecharge.

2. A method according to claim 1, in which said flowable metal is aparticulate metal constituted by prer'educed iron pellets or steelballs.

3. A method according to claim 2, in which starting is effected by aretractable auxiliary conductive electrode initially bridging the gapbetween the electrode tip and the charge so as to promote breakdown.

4. A method according to claim 2, in which starting is effected byprojecting an ionisable seeded fluid on to the gap between the electrodetip and the charge so as to promote breakdown.

5. A method according to claim 1, in which the method is performed in avacuum-arc re-melting or refining furnace and, the flowable metal is aparticulate metal and constitutes the whole of the ultimate vesselcharge.

6. A method according to claim 2, in which a reducing gas is passedthrough the particulate metal in the electrode.

7. A method of steelmaking by melting a charge in an electric furnace,comprising: continually dispensing a flowable metal on to the chargethrough a nozzle on the tip of a tubular non-consumable electrode,striking and sustaining an arc in the metal flowing between theelectrode nozzle and the surface of the charge, and correlating theelectrical operating parameters,

the physical parameters of the electrode, and

the flow rate of the metal so as to achieve a blanced condition in whichthe arc runs from just below the nozzle tip to the charge surface,whereby tomelt the charge and to reduce erosion of the nozzle tip.

8. A method according to claim 7, in which said flowable metal is aparticulate metal dispensed on to the charge, and

a reducing gas is passed through the particulate metal and starting iseffected by projecting an ionisable seeded fluid on to the gap betweenthe electrode tip and the charge so as to promote breakdown.

9. A method according to claim 8, in which said electrode constitutes areservoir for said flowable metal, and said flowable metal in saidelectrode provides a current conducting path between said electrode andthe metal flowing from said nozzle.

10. Electric furnace steelmaking apparatus comprising a tubular, currentconducting, non-consumable electrode constituting a reservoir forflowable, current conducting metal, and a non-current conducting nozzleat the tip of the electrode defining an orifice through which the metalis continuously dispensed on to the charge in the furnace, the flowablemetal in said electrode providing a current conducting path between thecurrent conducting electrode and the metal flowing from said nozzlewhereby an arc may be struck and sustained in the metal flowing betweenthe electrode nozzle and the surface of the charge for melting saidcharge.

11. Steelmaking apparatus according to claim 10, wherein said flowablemetal is a particulate metal and wherein the orifice in the nozzle tipis circular and has a diameter at least eight times the mean diameter ofthe particles.

12. Steelmaking apparatus according to claim 11, wherein an auxiliaryconductive electrode is mounted within the tubular electrode, theauxiliary electrode being movable towards and away from the charge forinitially bridging the gap between the electrode tip and the charge tostrike an arc. 13. Steelmaking apparatus according to claim 10, whereinthe electrode is made from an electrically conductive metal, and whereinthe nozzle tip is made from a refractory material. 14. Apparatusaccording to claim 10, wherein the electrode is of compositeconstruction embodying both electrically conductive and heat insulatingrefractory sleeves, and wherein the nozzle tip is made from a refractorymaterial. 15. Apparatus according to claim 10, wherein the electrodecomprises an inner heat insulating refractory sleeve, an electricallyconductive sleeve surrounding said refractory sleeve, and meansconnecting said electrically conductive sleeve with the interior of saidtubular electrode.

16. Apparatus according to claim 10, wherein said flowable metal isliquid metal.

17. A method according to claim 1, in which said flowable metal isliquid metal.

1. A method of steelmaking by melting a charge in an electric furnace,comprising: continually dispensing a flowable metal on to the chargethrough a nozzle on the tip of a tubular, nonconsumable electrode, andstriking and sustaining an arc in the metal flowing between theelectrode nozzle and the surface of the charge thereby melting thecharge.
 2. A method according to claim 1, in which said flowable metalis a particulate metal constituted by prereduced iron pellets or steelballs.
 3. A method according to claim 2, in which starting is effectedby a retractable auxiliary conductive electrode initially bridging thegap between the electrode tip and the charge so as to promote breakdown.4. A method according to claim 2, in which starting is effected byprojecting an ionisable seeded fluid on to the gap between the electrodetip and the charge so as to promote breakdown.
 5. A method according toclaim 1, in which the method is performed in a vacuum-arc re-melting orrefining furnace and, the flowable metal is a particulate metal andconstitutes the whole of the ultimate vessel charge.
 6. A methodaccording to claim 2, in which a reducing gas is passed through theparticulate metal in the electrode.
 7. A method of steelmaking bymelting a charge in an electric furnace, comprising: continuallydispensing a flowable metal on to the charge through a nozzle on the tipof a tubular non-consumable electrode, striking and sustaining an arc inthe metal flowing between the electrode nozzle and the surface of thecharge, and correlating the electrical operating parameters, thephysical parameters of the electrode, and the flow rate of the metal soas to achieve a blanced condition in which the arc runs from just belowthe nozzle tip to the charge surface, whereby to melt the charge and toreduce erosion of the nozzle tip.
 8. A method according to claim 7, inwhich said flowable metal is a particulate metal dispensed on to thecharge, and a reducing gas is passed through the particulate metal andstarting is effected by projecting an ionisable seeded fluid on to thegap between the electrode tip and the charge so as to promote breakdown.9. A method according to claim 8, in which said electrode constitutes areservoir for said flowable metal, and said flowable metal in saidelectrode provides a current conducting path between said electrode andthe metal flowing from said nozzle.
 10. Electric furnace steelmakingapparatus comprising a tubular, current conducting, non-consumableelectrode constituting a reservoir for flowable, current conductingmetal, and a non-current conducting nozzle at the tip of the electrodedefining an orifice through which the metal is continuously dispensed onto the charge in the furnace, the flowable metal in said electrodeproviding a current conducting path between the current conductingelectrode and the metal flowing from said nozzle whereby an arc may bestruck and sustained in the metal flowing between the electrode nozzleand the surface of the charge for melting said charge.
 11. Steelmakingapparatus according to claim 10, wherein said flowable metal is aparticulate metal and wherein the orifice in the nozzle tip is circularand has a diameter at least eight times the mean diameter of theparticles.
 12. Steelmaking apparatus according to claim 11, wherein anauxiliary conductive electrode is mounted within the tubular electrode,the auxiliary electrode being movable towards and away from the chargefor initially bridging the gap between the electrode tip and the chargeto strike an arc.
 13. Steelmaking apparatus acCording to claim 10,wherein the electrode is made from an electrically conductive metal, andwherein the nozzle tip is made from a refractory material.
 14. Apparatusaccording to claim 10, wherein the electrode is of compositeconstruction embodying both electrically conductive and heat insulatingrefractory sleeves, and wherein the nozzle tip is made from a refractorymaterial.
 15. Apparatus according to claim 10, wherein the electrodecomprises an inner heat insulating refractory sleeve, an electricallyconductive sleeve surrounding said refractory sleeve, and meansconnecting said electrically conductive sleeve with the interior of saidtubular electrode.
 16. Apparatus according to claim 10, wherein saidflowable metal is liquid metal.
 17. A method according to claim 1, inwhich said flowable metal is liquid metal.